The present invention is directed to lead-acid batteries, and particularly to lightweight, high energy batteries formed with electrodes having a non-lead conductive substrate covered by a continuous, non-corrosive, conductive coating.
Lead-acid batteries conventionally include a multiplicity of cells connected together in series. Each cell consists of a stack of alternating electrodes, namely cathodes and anodes. Often there is a layer of insulation between the electrodes. The cells are flooded or provided in some manner with an electrolyte (generally sulfuric acid).
In the past, the electrodes have been formed primarily of lead castings, stampings, or expended mesh of lead or of a lead alloy which provides the structural element to support the active material (lead) of the electrode. When charged, the electrodes become positively or negatively charged, where the energy is stored, until used in whatever application the battery is put. The battery may also be recharged from time to time.
Lead has been predominately used in such batteries for a long period of time. While lead is not particularly a good conductor of electricity, it is inherently corrosive resistant to the electrolytic acids. Other, more conductive metals are either too expensive to be used as the electrode for lead-acid batteries, or else they are quickly corroded during the charging action by the electrolytic acids. Therefore lead has remained as the predominant material. Lead is also very heavy, and in applications where weight is a factor, other alternatives have long been sought.
For example, in the aircraft industry, experts have calculated that the fuel cost of flying a commercial airliner is more than $3,000 per year per pound of weight flown. Therefore, if the airplane carries batteries having lead plates, considerable sums of money could be saved per plane if a lighter weight plate material could be found.
In previous attempts, one approach has been to plate lead onto other more conductive metals or metal alloys such as aluminum and copper. Copper is sixteen times as conductive as lead and weighs only about 70% as much. Aluminum, on the other hand has a specific gravity of only 20%-25% of lead and approximately eight times the conductivity of lead. Obviously, from the standpoint of weight and conductivity, copper and aluminum are good candidates to replace lead as the substrate for electrodes. However both materials are very susceptible to corrosion in the presence of sulfuric acid, and cannot be used as the positive electrode in a lead acid battery if left unprotected. Either material can be used as the negative electrode, and copper has in the past. In previous attempts to use aluminum or copper as the structural element for the plates of a lead acid battery in the past, attempts have been made to plate lead coatings onto aluminum or copper substrates. The conventional manner for plating lead is from an aqueous solution. The problem arises that when lead is plated from an aqueous solution, for one reason or another, the coatings are porous, and the sulfuric acid will quickly penetrate the coatings and attack the aluminum or copper. In such instances, the copper and aluminum plates have not survived the charging operation.
Some early research suggests that providing a lead coating over aluminum from a fused salt bath could provide a protective coating for aluminum plates and might be used as the electrodes of a lead-acid battery.
The present invention is directed toward reducing the weight per unit mass of the battery by replacing the lead or lead alloy plates of the battery with a lighter weight, conductive material that is plated by lead or some other conductive coating that is resistant to the electrolytic acid. Toward this end, then, the present invention utilizes a highly conductive non-lead substrate as the structural material for lead-acid battery plates. This substrate is significantly lighter than lead having a specific gravity of no greater than 70% that of lead. The substrate is then coated with a continuous layer of a conductive material that is corrosive resistant to the electrolytic acid to be used in the battery. This protective layer is plated on to the substrate from a fused salt bath. The result is a continuous coating that is substantially non-porous and protects the lighter weight conductive substrate. As a result, the energy to weight ratio, when compared to conventional lead plate cells, is in the range of 35-50 Watt-hours/kologram (WH/kg). That is to say, rather than an energy to weight ratio of approximately 30 WH/kg as in the case of conventional lead plate lead-acid batteries, the energy to weight ratio of batteries of the present invention may be in the range of about 35-50 WH/kg.
The non-lead substrates may be aluminum, aluminum alloys, aluminum/magnesium alloys, copper, copper alloys, nickel, nickel alloys or non-metallic materials such as graphite, carbon fibers and conductive plastics.
The corrosive resistant protective layer is formed from such materials as lead, lead alloys, lead/tin alloys and conductive epoxies. The fused salt bath may be selected from the group consisting of lead chloride alone; lead chloride, lithium chloride and potassium chloride; lead chloride and a lead/tin alloy; lead chloride, potassium chloride, and sodium chloride; lead chloride and lead nitrate; lead chloride and potassium nitrate; and lead chloride and sodium nitrate.
In an even more preferred embodiment, a thin intermediate layer (0.0001-0.0050 inches) of nickel, gold or titanium may be applied to the substrate, as a striking layer. This striking layer improves the bonding of the lead to the substrate as lead does not generally bond well directly to aluminum.